Strictly speaking, controllable drilling parameters (WOB, RPM and H) are not optimized in the state of art drilling practices. Drillers operate within a range of the value of drilling parameters recommended by service companies or manufacture of bit or rig systems based on lab tests or previous field experience. Experienced drillers may drill faster and safer than less experienced ones, because of the choice of a more proper set of values of drilling parameters in response to drilling dynamic phenomenon observed while drilling. The efficiency of drilling matters a greater deal, and the cost difference between more and less efficient drilling can be enormous. Assuming the cost for facility time of one day being 1% of the capital investment of the rig, a $500 M rig can cost up to $5 M per day for the expense in facility time alone, regardless of drilling status. The difference in time spent on drilling is magnified when, as is typical, average down-hole time begins to increases exponentially with the true vertical depth (TVD) of the wells, due to the increased rock strength and the general association of a harsher environment, such as higher temperature and pressure. Correspondingly, rate of penetration (ROP) gets slower as drilling goes deeper and deeper. Reports indicate that drilling the last 10 percent of the hole can account for ˜50% of the drilling cost. As a result, the difference in cost generated between higher and lower efficient drilling is more significantly different in deep drilling environments.
Although higher drilling efficiency is most evidently marked as achieving greater ROP, energy impact in drilling holds the constraint for drilling efficiency. If higher ROP is achieved at the cost of extremely high energy spending—resulting in adverse impacts on well bore stability, or low life time of bit and/or other rig components, or damage to the bottom hole assembly calling for frequent stoppage for parts replacement—the overall result can be less economical. As a result, drillers tend to evaluate the drilling performance in terms of not only ROP, but also the energy used in drilling, or the ratio of ROP to specific energy. In addition, energy consumption in drilling has found to be related to ROP in a way that an optimal condition may exists that can yield both high drilling efficiency while keep energy as low as possible.
The study of the effect of energy spending on drilling efficiency started about a half century ago and has continued as hydrocarbon resources in the shallow formations are depleted and gas and oil industries are forced to drill deeper for nonconventional resources. As early as 1965, Teale (“The Concept of Specific Energy in Rock Drilling,” Int. J. Rock Mech. Mining Sci. 2 (1965)) defined a term called Mechanical Specific Energy (MSE) describing the energy spent in removing a unit volume of rock/formation mass. Based on energy balance, he developed an equation which expresses MSE as function of WOB, ROP, RPM, and Torque. In Teales' study, it is realized that in most cases, MSE decreases with increases in Depth of Cut (DOC), the increment of drilling depth per revolution of a drill bit. By definition, DOC equals the ratio of ROP to RPM. As a result, for given RPM, MSE decreases with an increase in ROP. This indicates that it is not in conflict to achieve higher ROP with lower MSE.
Teale's finding has been expended in improving drilling efficiency. It has been realized that within certain range, ROP can be increased, and at the same time MSE reduced by increasing WOB. Attempts have also been made to increase ROP by increasing RPM [Remmen, Stephen M., Witt, Joseph W., Dupriest, Fred E., Implementation of ROP Management Process in Qatar North Field, SPE/IADC 105521 (2007).)]. This will usually hold since ROP is the product of DOC and RPM, and therefore ROP should increase linearly with RPM as long as DOC is kept constant. However, it is not always the case, because one may not be able to keep DOC constant all the time while increasing RPM. On the other hand, increasing ROP solely through increasing RPM while keeping DOC constant does not necessarily result in decrease in MSE since, according to Teale's work, a decrease in MSE while increasing ROP is caused mainly by an increase in DOC. In other word, solely increasing RPM seems not on the right track to achieve drilling optimization.
Complicated situations do exist where ROP and MSE vary with WOB and RPM in a very different manner respectively. Such factors as vibration and formation heterogeneity contribute to the complication of drilling dynamics. Complicated drilling dynamics may be reflected as nonlinear or even adverse effect of stochastic nature in MSE˜DOC and ROP˜WOB, or even ROP˜RPM relationship. One of the major factors causing the above complication is the balling or bottomhole jam phenomenon (Founder point). Greater WOB or RPM could generate more cuttings which changes the equilibrium of mass transfer in drilling dynamics, resulting in less DOC, and therefore less ROP and higher MSE. Increasing drilling fluid flow or temporally reducing WOB may sometimes efficiently remove excess amount of cuttings and bring the dynamic system to normal status at which the above mentioned relationship can be kept.
With the awareness of the role of hole cleaning effects of drilling fluid flow on drilling efficiency, a term of hydraulic power driving drilling fluid flow attracts attention in considering energy balance of drilling dynamics. Recently, Miguel Armenta [Miguel Armenta, SPE, Shell EPT-WT “Identifying Inefficient Drilling Conditions Using Drilling—Specific Energy”, SPE 116667 paper presented at the 2008 SPE Annual Technical Conference and Exhibition held in Denver, Colo.] expended Teale's expression by including hydraulic power used to drive drilling fluid as an additional energy term, and the total specific energy is named Drilling Specific Energy (DSE), in distinguishing from the term of Mechanic Specific Energy (MSE). In this application, both MSE and DSE are referred to as Specific Energy (SE).
Along with the progress in fundamental understanding of drilling dynamics, drillers have been generally striving to developing methods aimed at the goal of keeping MSE as low as possible and Rate of Penetration (ROP) as high as possible, by varying Weight On Bit (WOB), rotational speed (RPM) of the bit, and mud flow within normal operating limits while a variety of tests are conducted to optimize performance. A common method is the drill rate test, where various weight on bit (WOB) and RPM settings are utilized and results are observed to determine a combination generating the highest ROP. Another method is the drill-off test where a drill string is anchored and WOB and ROP are measured to determine a founder point, which is taken as the optimum WOB. See e.g., U.S. Pat. No. 7,857,047 issued to Remmert, et al. Additionally, a number of real-time systems exist which display an MSE calculated, so that operators may respond to indications outside of desired MSE ranges. See e.g., U.S. Pat. No. 7,243,735 issued to Koederitz et al., and see U.S. Pat. No. 7,938,197 issued to Boone et al., among others. In these systems, generally, an MSE is determined based on various measured parameters and an indication that a pre-determined MSE limit has been exceeded is provided, and the operator is expected to adjust WOB and RPM in order to clear the limit. In practice, this treats the WOB, RPM, and other parameters impacting the MSE as essentially independent from one another.
It is apparent that, due to the lack of direct connection between MSE and ROP with using only controllable drilling parameters, the current technology remains largely a trial and error method in searching for optimal conditions.
The manner in which the MSE is determined in these systems limits the effectiveness of this after-the-fact corrections actions. One of the drawbacks of performing trial and error searches is that once the inevitable over adjustment occurs (where the system has passed the optimal point), bottom hole jam will be so severe that the drillers often fail to bring the system off the founder point by merely reducing the one or more of the over tuned controllable drilling parameters and the entire search process for the optimal point may have to reset to the original begin-to-search status. Another drawback with trial and error methods is that they are time consuming, while the whole purpose of drilling optimization is to save time, since it is precious in drilling as mentioned earlier. The third problem with the trial and error method is that, most of the time the process is operating under not optimized conditions, because all the judgment on the drilling status is based on comparison between tested conditions, and none knows whether the next change is a better move.
Clearly, a critical technology barrier for optimizing drilling condition is that no direct relations have been established that relate the performance evaluating parameters (SE, and ROP etc.) directly with drilling parameters that can be adjusted conveniently by the driller while drilling. WOB, RPM and hydraulic power, H, are such parameters.
Currently available equations all involve intermediate parameters such as DOC and Torque (Tor), which themselves are not a directly controllable but rather a dependent of controllable parameters. Teale's equation involves both DOC and Tor. The ROP expression contains DOC. However, a relationship between either DOC or Torque with the independently controllable drilling parameters, i.e., WOB, RPM, and H, has not be established, even though one can claim that at some fixed condition—such as in well controlled lab or field drill-off test—empirical relations could be provided to describe these relations, the relations are limited to given rock type, controlled drilling fluid flow and type, known bit status, and well controlled cutting removal status, etc. However, in most real drilling conditions, all the above conditions will only be maintained for very short period of time. As the result, the empirical relationship developed as such will become invalid. In fact, so far the instructive guidance on drilling conditions provided based on lab or previous field tests has been limited to giving applicable ranges of controllable drilling parameters rather than an optimal point of operation. Strictly speaking, the state of the art is unable to perform a real time optimization of controllable drilling parameters due to the lack of a global understanding of the needed relationships between MSE and ROP with controllable drilling parameters.
Provided here is a method for determining the values of the controllable parameters WOB, RPM, and H which generate a most favourable outcome of drilling performance as expressed in terms of the performance evaluating parameters as selected from either a SE, ROP or the ratio of ROP to a SE. The optimization is realized through establishment of in-situ and direct relationship between MSE and ROP to controllable drilling parameters (WOB, RPM and H) respectively for an expected depth of well to be reached within the valid range of the relationship. Being aware of the stochastic component in the response of the dependent parameters to controllable drilling parameters, the molded relationship is considered to be valid with acceptable error bar and for a short period of time during which drilling conditions have not varied beyond the valid range. The molded relationship grasps the transit and abrupt feature of drilling dynamics by timely updating the model parameters describing the model relationship. The updating of the parameters specifying the model is performed periodically, real time, along the drilling process. The method receives a data stream from a Measure-While-Drilling (MWD) system and determines the response of both the intermediate parameters, DOC and Torque, respectively, to the changes of controllable drilling parameters WOB, RPM and H at a targeted point of length to be drilled (LD). Artificial neural networks (ANN) are used to model the relationship between DOC and controllable drilling parameters and between Torque and controllable drilling parameters respectively. MSE is calculated using the Teale's equation, with known controllable parameters and intermediate parameters determined through ANN simulation. DSE is also calculated consequently using an equation slightly different from proposed in the literature. To ensure the accuracy and validation of the ANN models for DOC and Torque, in certain embodiments, data no longer representing current drilling conditions are identified and removed in timely manner from the list of data used to train the model.
The establishment of expressing performance evaluating parameters directly in terms of controllable drilling parameters allows computerized optimization to select the best set of controllable drilling parameter values that leads to the most favorable value of performance evaluating parameters, for example, a lowest SE, or a highest Rate-of-Penetration (ROP) to SE ratio, as selected by the driller. The apparatus and method provides a display indicating the relationship of SE and/or ROP to the controllable parameters, based on modeling of data groups relayed by the MWD system during the drilling operation. Continuous sampling and modeling of MWD data during the drilling operation makes it possible for an operator to maintain drilling in the most efficient and safe manner by adjusting the controllable parameters.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.